Hitherto, control of an electrocoating bath by an ion exchange resin has been carried out by a column method, in which the ion exchange resin is densely filled into the column, so excess counter-ions in the bath are removed, and electrocoating resinous vehicle solubilized by the counter-ion is coagulated in the column. Though many improvements have been proposed for the control of such an elimination of the excess counter-ion in the column method (i.e. Japanese Patent Publication Nos. 23655/1973 and 40457/1973), they are far from a satisfactory solution of the above problems.